An accepted intervention for the treatment of cataracts is surgical removal of the lens and replacement of the lens function by an artificial intraocular lens (IOL). In the United States, the majority of cataractous lenses are removed by a surgical technique called phacoemulsification wherein the lens is divided into pieces and removed. Prior to removing the cataractous lens, an opening, or rhexis, must be made in the anterior capsule, a thin membrane that surrounds the lens. Currently, this opening is created by manually tearing the membrane with a surgical instrument. During phacoemulsification, there is a great deal of tension on the cut edges of the anterior capsulorhexis while the lens nucleus is emulsified. Accordingly, a continuous cut or tear ('rhexis), without “tags,” is a critical step in a safe and effective phacoemulsification procedure.
If the capsule is opened with numerous small capsular tears, the small tags that remain can lead to radial capsular tears, which may extend into the posterior capsule. Such a radial tear constitutes a complication since it destabilizes the lens for further cataract removal and safe intraocular lens placement within the lens capsule later in the operation. Further, if the posterior capsule is punctured, then the vitreous may gain access to the anterior chamber of the eye. If this happens, the vitreous must be removed by an additional procedure with special instruments. The loss of vitreous is also associated with an increased rate of subsequent retinal detachment and/or infection within the eye. Importantly, these complications are potentially blinding.
Conventional equipment used for phacoemulsification includes an ultrasonically driven handpiece with an attached oscillating tip. In some of these handpieces, the operative part is a centrally located, hollow resonating bar or horn directly attached to a set of piezoelectric crystals. The crystals supply ultrasonic vibration for driving both the horn and the attached cutting tip during phacoemulsification. Prior art devices and methods used for the capsulorhexis procedure require a great deal of skill on the part of the surgeon to produce a continuous curvilinear capsular opening. This is due to the extreme difficulty in controlling the path of the cutting tip of the device. For example, a typical procedure begins with a capsular incision made with a cystotome, i.e., a cutting tip as described above. This incision is then coaxed into a circular or oval shape by pushing the leading edge of the incision in the capsule, using the cystotome as a wedge rather than as a cutting device. Alternatively, the initial capsular incision may be torn into a circular shape by grasping the leading edge with fine caliber forceps and advancing the cut. Either of these approaches involves a very challenging maneuver and the tearing motion can sometimes lead to an undesirable tear of the capsule toward the back of the lens (i.e. toward the posterior capsule), even in the most experienced hands.
Moreover, even if a smooth capsular opening without tags is ultimately produced, the size and/or position of the capsular opening may present a problem. For instance, a capsular opening that is too small can impede the safe removal of the lens nucleus and cortex and prevent proper intraocular lens insertion into the lens capsule. The additional stresses necessary to accomplish the operation with a small or misplaced capsular opening put the eye at risk for zonular and capsular breakage. Either of these complications will likely increase the length and complexity of the operation and may result in vitreous loss.
A continuous, properly positioned, and circular opening is thus highly desirable because it results in: (1) a significant reduction in radial tears and tags within the anterior capsule, (2) capsule integrity necessary for proper centering of a lens implant; and (3) safe and effective hydrodissection. In addition, the capsulorhexis should be properly dimensioned relative to the diameter of the IOL being implanted in order to reduce the chances of a secondary cataract, also called posterior capsule opacification (“PCO”) and for use with proposed novel accommodative IOL designs that require close contact with the capsule. Therefore, there is a continuing need for an improved device for performing a continuous curvilinear capsulorhexis.